This event is the fifth mass bleaching on the reef in the past eight years.

Australia’s spectacular Great Barrier Reef, which stretches for some 2,300km (1,429 miles) off the country’s northeastern coast, is experiencing its worst bleaching event on record, the country's reef authority reported on Wednesday.

Often dubbed the world’s largest living structure, the Great Barrier Reef is home to a stunning array of biodiversity including more than 600 types of coral, 1,625 fish species, and 4,000 different molluscs.

But aerial surveys conducted by scientists show about 730 out of more than 1,000 reefs spanning the Great Barrier Reef have bleached, the Great Barrier Reef Marine Park Authority said.

“The cumulative impacts experienced across the reef this summer have been higher than previous summers,” the federally funded Marine Park Authority said in a statement.

The extent of the bleaching was revealed after the government agency tasked with the reef’s management confirmed early last month that the reef had been hit by its fifth major bleaching event since 2016.

The bleaching, when corals expel the colourful microscopic algae that live in their tissues in an effort to survive, was triggered by an increase in water temperatures that began last December.

“This prolonged heat exposure has caused mass bleaching of coral reef communities observed within all three regions of the Great Barrier Reef,” the authority said in an update on its website on Wednesday.

“The combination of aerial and in-water surveys in 2024 confirm a mass bleaching event, with prevalent and extreme bleaching observed at multiple reefs in all 3 regions of the Great Barrier Reef Marine Park.”

Bleaching occurs when coral expel the microscopic algae known as zooxanthellae to survive. If high temperatures persist, the coral can eventually turn white and die.

Reefs provide protection for coastal communities and are natural carbon sinks. Climate change poses the biggest threat to their survival because of their sensitivity to heat.

“Only the strongest and fastest possible actions to decrease global greenhouse gas emissions will reduce the risks of thermal stress on the Reef and limit the impacts from climate change on the Great Barrier Reef,” the marine park authority said.

One of the worst-impacted areas of the Great Barrier Reef this month is the Lizard Island, a small slice of tropical paradise off Australia’s northeast tip that is usually teaming with vibrant coral life..

Marine biologist Anne Hoggett, who has lived and worked on Lizard Island for over 30 years, said when she first arrived, coral bleaching only occurred every decade or so. 

Now, it is happening every year, she said, with about 80 percent of vulnerable Acropora corals on the Lizard Island reef suffering bleaching this summer.

Corals can recover from bleaching, and the Marine Park Authority said the full impact of the event would not be known for some time. It added that in-water surveys would continue.

Australia has invested about Aus$5 billion ($3.2 billion) into improving water quality, reducing the effects of climate change, and protecting threatened species.

The country is one of the world’s largest gas and coal exporters and has only recently set targets to become carbon neutral.

Whether these efforts will be enough for the reef to keep its World Heritage Status will be examined by UNESCO later this year.